Matching items in different arrays and performing operations on them

Question

I have a data structure that uses a Dictionary<ID,TimeSeries> to hold time series for different products. TimeSeries is implemented as something holding an array of Point, which contain a DateTime and a double.

Its interface is:

TimeSeries{
    int Length{ get; }
    Point ElementAt(int index);

    DateTime StartTime { get; }
    DateTime EndTime { get; }        
}

Given a pair of elements of such data structure I need to find the first time instant where they diverge. For my definition of divergence, if a dictionary contains an element and the other doesn't I need to return the time of the first point of the TimeSeries I have.

This is the code I'm using to compute the difference and that I'd like you to review and comment:

bool TryFindFirstDivergenceTime(Dictionary<ID, TimeSeries> baseline, Dictionary<ID, TimeSeries> other, out DateTime firstDivergenceTime)
{
    firstDivergenceTime = default(DateTime);

    foreach(var baselineIDSeriesPair in baseline)
    {
        if(other.ContainsKey(baselineIDSeriesPair.Key))
        {
            firstDivergenceTime = Min(FindFirstDivergenceTime(baselineIDSeriesPair.Value, other[baselineIDSeriesPair.Key]), firstDivergenceTime);
        }
        else
        {
            firstDivergenceTime = Min(baselineIDSeriesPair.Value.Start, firstDivergenceTime);
        }
    }
    foreach(var otherIDSeriesPair in other)
    {
        if(!baseline.ContainsKey(otherIDSeriesPair.Key))
        {
            firstDivergenceTime = Min(otherIDSeriesPair.Value.Start, firstDivergenceTime);
        }
    }

    return firstDivergenceTime != default(DateTime);
}

public DateTime Min(DateTime first, DateTime second)
{
    if(first == default(DateTime))
        return second;
    if(second == default(DateTime))
        return first;
    return new DateTime(Math.Min(first.Ticks, second.Ticks));
}

DateTime FindFirstDivergenceTime(TimeSeries first, TimeSeries second)
{
    if(first.Length != second.Length)
    {
        throw new ArgumentException();
    }
    for(var i=0; i < first.Length; i++)
    {
        if(! first.ElementAt(i).Equals(second.ElementAt(i)))
        {
            return Min(first.ElementAt(i).Time, second.ElementAt(i).Time);
        }
    }
    return default(DateTime);
}

I have the feeling that I am not approaching the problem in the right way as the code contains the repetition of the logic to find the time of the first difference when I don't have the same product in both the dictionaries.

The other think that I don't particularly like is that I find myself breaking th symmetry and using for on dictionary Key and Value from its IEnumerable, while from the other I need to perform a lookup.

How would you address those issues?

Answer 1

Before getting to your question about reducing the duplication, there's another issue I'd like to point out first. The use of default(DateTime) as a check for the absence of a value is incorrect and misleading. default(DateTime) is a valid date. However unlikely it is that you will have a date that equals that value, it is misleading to use because anyone looking at your code that sees a DateTime variable will assume that it holds a proper date value because it has to. As a struct, it can't be represented as null, so it always holds a value. Enter nullable types. DateTime? clearly expresses that the variable can be in a state where it doesn't hold a date value.

That said, it appears you're trying to join the two dictionary and compare in these ways:

1. When there are matching keys.
2. When only one of the dictionaries holds a key.

Sounds like a Full Outer Join, which someone has kindly shared a Linq-esque extension method for over on Stack Overflow. Using this, you can concisely and expressively rewrite your methods like so to avoid duplication (note the use of DateTime?):

// Note: I think this is a bad method name (it's the same as the one below,
// which I think is better suited for the name). It should probably express
// the comparison between these Dictionary maps.
public static DateTime? GetEarliestDivergenceTime(Dictionary<int, TimeSeries> baseline, Dictionary<int, TimeSeries> other, DateTime? defaultEarliestDivergenceTime = null)
{
    // Omitting 'FullOuterJoin' code in StackOverflow link:
    var earliestDivergenceTimes = baseline.FullOuterJoin(
        other,
        b => b.Key,
        o => o.Key,
        (b,o) => GetEarliestDivergenceTimeOrDefault(b, o));

    return earliestDivergenceTimes.Min() ?? defaultEarliestDivergenceTime;
}

public static DateTime? GetEarliestDivergenceTime(TimeSeries ts1, TimeSeries ts2, DateTime? defaultEarliestDivergenceTime)
{
    DateTime? earliestDivergenceTime = null;

    if(ts1 != null && ts2 != null)
    {
        // Compare.
    }
    else if(ts1 != null && ts2 == null)
    {
        // Use ts1.
    }
    else if(ts1 == null && ts2 != null)
    {
        // Use ts2.
    }

    // earliestDivergenceTime will be null if both t1 and t2 are null.
    return earliestDivergenceTime ?? defaultEarliestDivergenceTime;
}